For the following situations, identify the population, the sampling units, and the sampling plan. If it is a multistage sampling plan, identify the population, sampling unit, and sampling plan at each stage.

Problem 6. A geologist is interested in the surface geology of a certain area. He divides the area up with a grid into 20 equal-sized parcels. Within each parcel, he randomly selects 5 points and obtains measurements at each of these points.